Recently, as the mobility and portability of electrical and electronic devices have increased, the demand for secondary batteries has rapidly increased. Lithium secondary batteries started to be produced industrially by Sony Corp., Japan, in the beginning of the 1990s, and occupy the majority of the portable phone and notebook computer markets, because such lithium secondary batteries have advantages over prior Ni—Cd and Ni-MH batteries in that they have light weight and high capacity. Recently, such lithium secondary batteries have been increasingly used in high-output large-capacity batteries in electric power tools, electric bicycles, electric scooters, game machines, wireless cleaners, service robots, hybrid vehicles, etc.
Lithium ion secondary batteries generally include lithium cobaltate (LiCoO2) as a cathode active material, carbon as an anode active material, and lithium hexafluorophosphate as an electrolyte. As the cathode active material, lithium cobaltate (LiCoO2) and lithium nickelate (LiNiO2), having a layered structure, and lithium manganate having a spinel structure, are known, but lithium cobaltate is mostly used in practice for commercial purposes. However, because not only the supply and demand of cobalt as a main component is unstable, but also the cost of cobalt is high, materials obtained by partially substituting cobalt with other transition metals such as Ni and Mn, or spinel lithium manganate containing little or no cobalt, etc., started to be commercially used. Also, novel compounds showing high structural stability even under high voltages, or materials by doping or coating existing cathode materials with other metal oxides so as to have improved stability, have been developed.
Among prior methods of preparing cathode active materials, the most widely known methods include a dry calcination method and a wet precipitation method. According to the dry calcination method, a cathode active material is prepared by mixing an oxide or hydroxide of a transition metal such as cobalt (Co) with lithium carbonate or lithium hydroxide as a lithium source in a dry state, and then calcining the mixture at a high temperature of 700-1000° C. for 5-48 hours. The dry calcination method has an advantage in that it is easy to approach, because it is a technology which has been conventionally frequently used. However, it has shortcomings in that it is difficult to obtain single-phase products because it is difficult to mix raw materials uniformly, and in the case of multi-component cathode active materials consisting of two or more transition metals, it is difficult to arrange two or more elements uniformly to the atomic level. Also, in the case of using methods of doping or substituting cathode active materials with a specific metal component in order to improve electrochemical performance, there are problems in that the specific metal component added in small amounts is difficult to mix uniformly, and the loss thereof necessarily occurs a grinding and classifying process for obtaining particles having the desired size.
Another conventional method for preparing cathode active materials is the wet precipitation method. According to the wet precipitation method, a cathode active material is prepared by dissolving in water a salt containing a transition metal such as cobalt, adding alkali to the solution to precipitate the transition metal in the form of transition metal hydroxide, filtering and drying the precipitate, mixing the dried material with lithium carbonate or lithium hydroxide as a lithium source in a dry state, and then calcining the mixture at a high temperature of 700-1000° C. for 1-48 hours. The wet precipitation method is known to easily obtain a uniform mixture by co-precipitating two or more transition metal elements, but has problems in that it requires a long period of time for the precipitation reaction, is performed using a complicated process, and causes waste acids as by-products. In addition, various methods, including sol-gel methods, hydrothermal methods, spray pyrolysis methods and ion exchange methods, have been suggested as methods for preparing cathode active materials for lithium secondary batteries.
Meanwhile, methods of preparing LiCoO2 particles, and LiMn2O4 particles, etc., using supercritical water, have recently been reported (K. Kanamura, et al., Key Engineering Materials, 181-162 (2000), pp. 147-150). Japanese Patent Laid-Open Publication No. JP2000-72445A discloses a method of preparing a metal oxide for cathode active materials by allowing lithium ions to react with transition metal ions in a supercritical or subcritical state in a batch-type reactor. Also, Japanese Patent Laid-Open Publication No. JP2001-163700 discloses a method of preparing a metal oxide for cathode active materials by allowing lithium ions to react with transition metal ions in a supercritical or subcritical state in a batch-type reactor and a continuous reactor. According to the disclosure of such patent documents, in the case of the batch-type reactor, an increase in Li/Co ratio, an increase in alkali molar ratio, an increase in nitric acid concentration, and the addition of an oxidizing agent, lead to a decrease in the content of impurity CO3O4 and an increase in the content of single-phase LiCoO2. However, particles obtained according to the disclosure of such patents are not suitable for use as cathode active materials, because the purity of LiCoO2 in the particles is only a maximum of 97.8%. Also, in the case of using the continuous reactor, a metal oxide for cathode active materials is synthesized by continuously pumping an aqueous cobalt salt solution or an aqueous manganese salt solution under pressure into the reactor, adding supercritical water and hydrogen peroxide (H2O2) thereto, and then allowing the mixture to react in conditions of about 400° C. and about 300 bar. In this case, the reaction time is as relatively short as 30 seconds or less, but the synthesized product is known to have low purity and poor electrochemical properties. Also, when the above-described methods are used to prepare single metal oxide such as lithium cobaltate or lithium manganate, they will provide highly crystalline particles having a size as large as submicrons (μm). However, these methods have problems in that, when they are used to prepare a multicomponent metal oxide consisting of more than two components, they cannot synthesize crystals having excellent solid-solution stability because the crystallization rates of the components are different from each other, and also the synthesized particles are difficult to apply as cathode active materials because such particles are as excessively small as the nanometer scale. Thus, there is an urgent need to develop a novel cathode active material, which satisfies high performance and low cost requirements, and a preparation method thereof.